Allow custom info to be added to Help page

Fixes #2536

**Admin screen**

![image](https://gitlab.com/jeroenvanbaarsen/gitlab-ce/uploads/e62f3e0ff40558fb955f09e469293521/image.png)

**Text on the Help page**

![image](https://gitlab.com/jeroenvanbaarsen/gitlab-ce/uploads/ba315fb418e1698d8f44b88c7cc6c29a/image.png)

/cc @DouweM

See merge request !1309